Author: (Linnaeus, 1758)
Dorsal coloration dark blue, bright blue on sides and abruptly white on the undersides, usually slender body, long snout, large eyes, gillraker papillae, long, narrow, pointed pectoral fins, short labial furrows, first dorsal fin on back but closer to the pelvic bases than the pectorals, second dorsal less than a third size of first, a weak keel on the caudal peduncle, and a narrow-lobed caudal fin with a long ventral lobe.
Body rather slender. Head narrow, only moderately depressed, not trowel-shaped; snout narrowly parabolic in dorsoventral view, very long, with preoral length greater than internarial space and mouth width; eyes large, without posterior notches; spiracles absent; unique papillose gillrakers present on internal gill openings; nostrils small, internarial space about 2.5 to 3 times the nostril width; anterior nasal flaps very short and broadly triangular, not tubular; labial furrows very short with uppers shorter than lowers and with their ends falling far behind eyes; teeth well differentiated in upper and lower jaws; upper and anteroposteriors with broad, triangular, curved erect to oblique, serrated cusps but with no blades or cusplets (except in very young specimens); lowers with slender cusps, no blades or cusplets, and variable serrations; cusps of lower teeth not prominently protruding when mouth is closed; 24 to 31/25 to 34 rows of teeth. Interdorsal ridge absent; low dermal keels present on caudal peduncle; upper precaudal pit transverse and crescentic. First dorsal origin well behind pectoral rear tips, its midbase much closer to pelvic than to pectoral bases, and free rear tip slightly anterior to pelvic origins; second dorsal fin much smaller than first, its height 1/2 of first dorsal height or less; its origin slightly posterior to anal insertion; pectoral fins very narrow and somewhat falcate, pectoral length from origin to free rear tip 1/2 or less of pectoral anterior margin; pectoral origins varying from under interspace between third and fourth gill slits to under fourth gill slits; anal slightly larger than second dorsal, with slRort preanal ridges and a deeply notched posterior margin. Colour intense deep blue above, whitebelow, without a colour pattern. Large sharks, adults possibly reaching 4 m or more.
Oceanic and circumglobal in temperate and tropical waters (probably the widest ranging chondrichthyian): Western Atlantic: Newfoundland to Argentina. Central Atlantic. Eastern Atlantic: Norway to South Africa, Mediterranean. Indo-West Pacific: South Africa and southern Arabian Sea to Indonesia, Japan, Australia, New Caledonia and New Zealand. Central Pacific. Eastern Pacific: Gulf of Alaska to Chile.
Habitat and Biology:
A wide-ranging, oceanic-epipelagic and fringe-littoral shark, occurring from the surface to at least 152 m depth. Although an offshore species, it may venture inshore, especially at night, and often in areas with a narrow continental shelf or off oceanic islands. In temperate waters blue sharks occasionally venture to the edges of kelp forests or sufficiently far inshore to be caught in pound nets. The blue shark is often found in large aggregations, not tightly organized schools, and frequently close to or at the surface in temperate waters It prefers relatively cool water at 7 to 16 C but can tolerate water at 21 C or even more; it ranges far into the tropics but shows tropical submergence and occurs at greater depths there. In the tropical Indian Ocean the greatest abundance of blue sharks occurs at depths of 80 to 220 m, with temperatures about 12 to 25°C.
The blue shark is often seen cruising slowly at the surface, with its large pectoral fins outspread, and its first dorsal fin and terminal caudal lobe out of the water. When disturbed, hooked or attacking prey it is capable of bursts of speed; one was seen by the writer to jump partway out of the water when hookedon a pelagic longline. It will often circle a food item before moving in to devour it.
In the Pacific the blue shark is present in greatest abundance between 20 and 50 N, but in this area it shows strong seasonal fluctuations in abundanande, connected with yearly migrations northward in summer and southward in winter. In the tropics between 20 N and S it is uniformly abundant throughout the year. In the North Atlantic, tagging and recapturing of individuals has shown a regular clockwise trans-Atlantic migration route with the current system there. Sharks tagged off the USA have been recovered off Spain, in the Straits of Gibraltar, and in the equatorial north-central Atlantic, while sharks tagged in the Canary Islands have turned up off Cuba. Apparently the sharks ride the Gulf Stream to Europe, take various currents down the European and African coasts, and ride the Atlantic North Equatorial Current to the Caribbean region. There is considerable sexual segregation in populations, with females more abundant at higher latitudes than males.
Viviparous, with a yolk-sac placenta; number of young 4 to 135 per litter. The number of young varies considerably among females, more so than any other livebearing shark, and may be partially dependent on size of the female. In the Indian Ocean sex ratios of fetuses were in aggregate 1:1, though individual females often have slightly more of one sex than another. The gestation period is 9 to 12 months, and possible maximum age at least 20 years. Off the western North Atlantic most female blue sharks are immature at 0 to 4 years old, adolescent at 4 to 5 years, and adult from 5 to 6 years and beyond. Males mature at about 4 to 5 years of age. Unlike some other carcharhinids, clasper growth in males is apparently a prolonged and gradual process that may take at least a year, making the condition of claspers rather difficult to use for determining maturation of males. Females have a prolonged maturation phase in their fourth and fifth years during which timethey become sexually active and copulate with males. Five-year old females store sperm in their shell glands after the mating season in late spring to early winter, and retain it for a prolonged period while their ovaries and oviducts enlarge and become differentiated; in their sixth year, in the next spring, fertilization occurs and young are born in spring to early summer of their seventh year. Some females may mature a year earlier than the majority or shift out of phase with them in having young out of the usual season. Sharks in tropical areas may mate throughout the year.
Courtship behaviour and copulation has not been observed in the blue shark, but these apparently involve biting of females by males. Among adult and subadult sharks, this behaviour is sufficiently consistent with sex that sharks in the field can be sexed accurately merely by the presence or absence of bite wounds or scars. The blue shark has an unusual morphological adaptation for this behaviour; adolescent and mature females develop skin about three times as thick as males.
The blue shark feeds heavily on relatively small prey, especially bony fishes and squid, though other invertebrates, small sharks, and mammalian carrion is readily taken and seabirds occasionally are caught at the surface of the water. Much of the prey of the blue shark is pelagic, though bottom fishes and invertebrates figure in its diet also. Fish prey include herring, sardines and other clupeids, anchovies, conger eels, Pacific salmon, daggertooths (Anotopterus), lancetfish, needlefish, sauries, flyingfish and their eggs, pipefish, hake, cod, haddock, pollock, whiting and other gadoids, mullet, pomfrets, mackeral, damselfish, tuna (including bigeyes and yellowfins), jacks, remoras, sea bass, trunkfish, rockfish, spiny dogfish (Squalus acanthias) and, in one instance, a goblin shark (Mitsukurina).
Squid are a very important prey of these sharks; some species form huge breeding aggregations, which are attended by blue sharks. Some sharks slowly ingest masses of squid almost like browsing herbivores, and leisurely swim forward and sweep their heads and tails in broad arcs, catching squid at the corners of their mouths. Newly arrived and presumably hungry sharks may rapidly charge through dense schools of squid gulping down large masses. These sharks also feed on the undersides of dense schools, assuming a vertical posture and lunging upward into the school to take prey. With scattered or alert squid the sharks may attack in swift, fast turns and catch them in their mouth corners. Cuttlefish, vampire squid and pelagic octopi are also taken by blue sharks, as well as sea snails, slipper lobsters, shrimp, mantis shrimp and crabs (including swimming crabs). In the eastern North Pacific, masses of pelagic red crabs are readily eaten by these sharks. The papillose gillrakers of the blue shark, unique among the requiem sharks, may be very useful for preventing small prey like squid, red crabs or anchovies from slipping out the internal gill slits. On the other hand, these sharks proverbially gather in great numbers at a whale carcass and may feast avidly on its blubber in a feeding frenzy. Whale and porpoise blubber and meat have been recorded from blue shark stomachs.These sharks are also known to attack the cod-ends of trawls to remove the fish. Blue sharks have been seen biting at floating objects such as tin cans and boxes at the surface.
A dangerous species, with several attacks on people and boats on record. Spearfishing divers have been harassed by these sharks, and have had to fend them off with spears to keep from being bitten. Sometimes these sharks will slowly circle divers, possibly out of curiosity, in some instances for a quarter of an hour or more. An odd 'sport' for divers off southern California is swimming with blue sharks that have been baited into the vicinity of a boat, possibly as a test of virility on the part of the mostly male divers. The blue shark is not strongly aggressive under such circumstances of contact with people underwater, but on the other hand is not very timid. A slowly approaching shark of this species should be handled with caution, as it may bite (possibly in test-feeding) after circling for some time.
Maximum size 383 cm on reasonably good evidence, though unconfirmed reports of larger individuals up to 4.8 to 6.5 m are mentioned in the literature; males maturing between 182 and 281 cm, and reaching at least 311 cm, females adolescent at 173 to 221 cm, adult at 221 to at least 323 cm; size at birth about 35 to 44 cm.
A length/weight equation for the blue shark (Strasberg, 1958) is: Log Wt (lbs) = -5.396 + 3.134 log TL (cm).
Interest to Fisheries:
This common oceanic shark is usually caught with pelagic longlines but also hookandlines, pelagic trawls, and even bottom trawls near coasts. It is utilized fresh, smoked, and dried salted for human consumption; its hides are used for leather; fins for shark-fin soup base; and also for fishmeal and liver oil. This shark is also considered a game fish and taken by sports anglers with rod and reel.
Holotype: None. Type Locality: "Habitat in Oceano Europaeo".